Turbine shroud support structure



Sept. 1, 1964 w. M. FARRELL 3,146,992

TURBINE SHROUD SUPPORT STRUCTURE Filed Dec. 10, 1962 INVENTOR. lV/[l/flM 1% 1 48851! United States Patent TURBINE SHRGUID UPPORT STRUCTUREWiiiiam Miller Farrell, Scotia, N.Y., assignor to General ElectricCompany, a corporation of New York Filed Dec. 10, 1952, Ser. No. 243,5574 Claims. (Cl. 25339) This invention relates to a shroud ring structurefor a turbine and, more particularly, to an improved shroud ring supportstructure for providing desired clearances between the shroud ring andthe tips of the buckets of an associated turbine wheel.

Conventional shroud rings are supported in the turbine casing such thatthe casing and the shroud ring expand and contract as an integral unitin response to changes in operating temperatures. The casing and shroudring assembly, which has substantially less mass than the turbine wheel,responds to changes in turbine operating temperature at a more rapidrate than the turbine wheel. Due to the difference in expansion andcontraction rates, the clearance between the shroud ring and the buckettips varies under different operating conditions' Since it is desirableto prevent rubbing between the shroud ring and the turbine wheel, theturbine is generally designed to avoid rubbing at the most extremecondition of relative thermal expansion and contraction which may beencountered. The most severe condition occurs during a rapid shut-down,or, in the case of a gas turbine engine, throttle chop, when the shroudring contacts very rapidly. It is therefore necessary with conventionalshroud structures to provide a relatively large clearance at the normaloperating condition of the turbine in order to prevent rubbing during arapid shutdown. An abradable shroud ring is sometimes used to permitintentional rubbing during shut-down, but this expedient does not avoidincreased clearances during subsequent engine operation at the normaloperating temperatures.

With such conventional shroud ring structures, the clearance necessaryto prevent rubbing may be sufficient to cause poor efficiency byallowing excessive leakage of operating fluid around the bucket tips.Recognizing this problem, efforts have been made in the past to provideshroud ring support structures for maintaining both the desired minimumtip clearance at the normal operating condition of the turbine and aclearance suilicient to prevent rubbing during a rapid shut-down. Toobtain such clearance at the various operating conditions, the shroudring must be moved relative to the casing in an inward direction toprovide the minimum tip clearance at the normal turbine operatingcondition and in an outward direction to prevent rubbing during engineshut-down. In other words, the casing and shroud ring are no longerallowed to expand and contract as an integral unit. The clearances maybe obtained by means of a control system which positions the shroud ringin accordance with changes in a selected engine operating parameter suchas, for example, temperature. For certain engine applications, such acontrol system is undesirable, although it may be extremely accurate inmaintaining the desired clearances, since it greatly increases both theexpense of manufacturing the engine and the complexity of the completedengine. It also has been proposed in the past to obtain the desiredclearances by means of shroud support members of a material having ahigher coefiicient of linear expansion than the casing material. Thesupport members move the shroud ring inwardly relative to the casing inresponse to an increase in temperature as the expanding casing movesoutwardly relative to the bucket tips. Similarly, the shroud rin movesoutwardly relative to the casing when the temperature decreases. As apractical matter, however, it is difiicult to obtain the necessarymovement relative to the casing since the diameter of the casing is muchgreater than the space between the shroud ring and the casing in whichthe support members are conventionally located. Therefore, the supportmembers must have an extremely high coefiicient of expansion, which isnot easily attained, or must extend outwardly through the engine casingin order to obtain the length necessary for the desired movement. Inview of these difficulties, support members of this kind are notnormally used.

Accordingly, it is an object of this invention to provide means formaintaining desired clearances between a shroud ring and the tips of theassociated turbine buckets at difierent operating conditions of theturbine.

Another object of this invention is to provide means for maintainingboth a desired minimum tip clearance at the normal operating conditionof the turbine and a clearance suflicient to prevent rubbing during arapid turbine shut-down.

A further object of this invention is to provide simple and inexpensivemeans for mainta ning desired tip clearances which neither adds to theexpense of manufacturing the turbine nor increases substantially thecomplexity of the turbine.

Briefly stated, in accordance with an illustrated embodiment of theinvention, bimetallic thermal support strips are provided formaintaining desired clearances between a circumferentially extendingsegmented shroud ring and the tips of a row of turbine buckets. Thebimetallic support strips are supported by their ends in the spacebetween the segmented shroud ring and the casing, each strip beingpositioned with its layer having the lower coefficient of expansionadjacent the casing. The unsupported center portion of each bimetallicsupport strip is connected to a respective one of the shroud ringsegments. With increasing turbine operating temperature, the bimetallicsupport strips deflect to move the shroud ring segments inwardlyrelative to the casing. Similarly, the bimetallic support strips deflectto move the shroud ring outwardly relative to the casing with decreasingtemperature.

While the invention is distinctly claimed and particularly pointed outin the claims appended hereto, the invention, both as to organizationand content, will be better understood and appreciated, along with otherobjects and features hereof, from the following detailed descriptiontaken in conjunction with the drawing, in which:

FIGURE 1 is an end view of an axial flow turbine utilizing thisinvention;

FIGURE 2 is a fragmentary pictorial view of a shroud assembly includingas a part thereof the bimetallic support element of this invention;

FIGURE 3 is a view of the shroud assembly of FIG- URE 2 mounted in theturbine casing;

FIGURE 4 is a View similar to FIGURE 3 showing the shroud assembly in amoved position; and

FIGURE 5 is a View taken along line 5-5 of FIG- URE 4.

Referring now to the drawing, an axial flow turbine is illustrated inFIGURE 1. The turbine, which is of a lightweight type particularlysuited for use in aircraft gas turbine engines, has a casing 10 which ispreferably split as shown along a horizontal line 11 into halves inorder to facilitate assembly and disassembly of the turbine. The casinghalves are joined to form the unitary casing 10 by means of a flange andbolt connection indicated generally at 12. A turbine wheel 13 having aplurality of radially extending turbine buckets 15 secured to itsperiphery in a well known manner is rotatably mounted in the casing 16,the turbine wheel 13 driving a shaft 14. A shroud ring 16 comprised of aplurality of circumferentially extending arcuate segments 16', as bestshown in FIGURE 5, is supported in the casing 10 in circumferentiallyspaced relation to the tips of the turbine buckets 15. In thisdescription, the space 17 between the shroud ring 3,1 3 16 and the tipsof the turbine buckets will be referred to as either the clearance orthe tip clearance.

Referring now to FIGURES 2 through 5, each of the arcuate shroud ringsegments 16, which may be fabricated if desired from an abradablematerial such as the honeycomb material illustrated, is provided with abacking plate 20 having a mounting bracket 21 mounted thereon. At leastone, and preferably several as shown in FIGURE 3, bimetallic supportelements or strips 22 are secured to the mounting bracket 21 by rivets23 or other suitable fastening means. Each of the bimetallic supportstrips 22 is comprised of two layers 24 and 25 having differentcoeflicients of thermal expansion bonded or otherwise suitably joinedtogether so as to react as a unitary structure in response to changes intemperature. The bimetallic support strip 22 is fastened to the mountingbracket 21 by the rivets 23 with the layer 24 having the highercoefficient of thermal expansion adjacent the mounting bracket 21. Theaction of the bimetallic support strips 22 in response to changes intemperature will be described in detail at a later point in thisdescription.

Turning to FIGURES 3 and 4 in particular, it will be seen that theturbine casing 10 has circumferentially extending flanges 3t} and 31which extend radially inwardly from the casing 10 to form with thecasing a circumferential channel 32. The upstream flange and thedownstream flange 31 are provided with circumferentially extendinggrooves 33 and 34, respectively, opening into the channel 32. The shroudring segments 16' are positioned in the channel 32 with the ends of eachof the bimetallic support strips 22 received in the grooves 33 and 34.The backing plate 20 is slidably engaged between the downstream face 35of the upstream flange 30 and the upstream face 36 of the downstreamflange 31, the flanges thereby permitting radial movement of the shroudring segments 16 while preventing axial movement relative to thecenterline of the turbine.

As described previously, the bimetallic support strips 22 are comprisedof two layers 24 and 25 having different coefficients of thermalexpansion. The layers 24 and 25 are bonded or otherwise suitably securedtogether to react as a unitary structure in response to changes intemperature. Since the layers are not free to move relative to eachother, the support strip 22 will change its shape when heated or cooled.The support strip 22, its layers 24 and 25 having been secured togetherat a specific reference temperature to form a flat structure, will bendinto a curve whenever the temperature is varied from the referencetemperature, the transverse motion of the support strip 22 being verymuch larger than the change in length of either of the layers 24 and 25.The actual selection of materials for the layers 24 and 25 in anyparticular application will depend upon a number of factors such as, forexample, the amount of deflection desired and the ability of thematerials selected to withstand the high turbine temperatures.

Referring specifically to FIGURE 3, the arcuate shroud ring segment 16'is shown in its position for normal turbine operating conditions. Withthe layer 25 having the lower coefiicient of thermal expansion adjacentthe casing, the high turbine operating temperatures cause the bimetallicelement 22 to deflect into the position shown. With the ends of thesupport element 22 secured in the circumferential grooves 33 and 34,only the center of the element 22 is free to move, the bimetallicsupport element 22 thereby moving the shroud ring segment 16' inwardlyrelative to the casing with increasing temperature. With the shroud ringsegment 16 positioned as shown, the turbine operates efficiently sincethe clearance 17 between the shroud ring segment 16' and the tips of theturbine buckets 15 is quite small.

Assuming now that the turbine is rapidly shut-down, it will be clearthat the casing 10 will contract at a greater rate than the turbinewheel 13 because of its much smaller mass. If the shroud ring 16 wereconstrained to contract integrally with the casing 10 as in conventionalstructures, the shroud ring segments 16 would move into rubbing contactwith the tips of the turbine buckets 15. Even if the segments 16 arefabricated of abradable material as shown, inefficient operation wouldthereafter result because of increased tip clearance 17 at normaloperating conditions. The bimetallic support strips 22 of this inventioneliminate rubbing by moving the shroud ring 16 in response totemperature changes independently of the casing 10 and in an oppositedirection. When the temperature drops, the bimetallic support strips 22react to the change by deflecting to the flat position shown in FIGURE4, thereby increasing the tip clearance 17. The casing 10 can thereforecontract relative to the turbine wheel 13 without resulting in rubbingcontact between the shroud ring 16 and the tips of the turbine buckets15. When the turbine is again returned to its normal operatingcondition, the bimetallic support strips 22 will return the shroudsegments 16 to the position shown in FIGURE 3 to provide a minimum tipclearance.

It is thus seen that the bimetallic support elements of this inventionprovide simple and inexpensive means for maintaining both a desiredminimum tip clearance at the normal operating condition of the turbineand a clearance suflicient to prevent rubbing during a rapid turbineshutdown.

While the invention is particularly applicable for use in hightemperature turbines and has been so described, it will be obvious tothose skilled in the art that the invention may likewise be practiced inconnection with other turbomachines which are subjected to substantialtemperature variations, such as axial flow compressors. It will also beunderstood that the invention is not limited to the specific details ofconstruction and arrangement of the embodiment illustrated and describedherein. It is intended to cover in the appended claims all such changesand modifications which may occur to those skilled in the art withoutdeparting from the true spirit and scope of the invention.

What is claimed as new and desired to secured by Letters Patent of theUnited States is:

1. In a turbomachine, a cylindrical casing, a turbine Wheel rotatablymounted in said casing, a row of radially extending turbine bucketsperipherally mounted about said turbine wheel, a shroud ringcircumferentially surrounding said row of turbine buckets in spacedrelation thereto, said shroud ring comprised of a plurality of separatecircumferentially extending arcuate segments, a plurality of bimetallicsupport strips mounted in said casing, said bimetallic support stripseach comprised of two layers having different coefficients of thermalexpansion, means connecting the center portion of each of saidbimetallic support strips to a respective one of said arcuate shroudsegments, said bimetallic support strips disposed so as to deflect withtemperature changes to move said arcuate shroud segments inwardlyrelative to said casing with increasing turbine operating temperatureand outwardly with decreasing temperature.

2. In a turbomachine, a cylindrical casing, a turbine Wheel rotatablymounted in said casing, a row of radially extending turbine bucketsperipherally mounted about said turbine wheel, a shroud ringcircumferentially surrounding said row of turbine buckets in spacedrelation thereto, said shroud ring comprised of a plurality of separatecircumferential extending arcuate segments, first and secondcircumferentially extending flanges extending radially inwardly fromsaid casing and forming therewith a circumferential channel surroundingsaid shroud ring, a plurality of bimetallic support strips, saidbimetallic support strips, said bimetallic support strips each comprisedof two layers having different coefficients of thermal expansion, eachof said bimetallic support strips mounted between said flanges so thatthe center of the support strip spans said channel with the layer havingthe lower coefficient of thermal expansion adjacent said casing, meansconnecting the center portion of each of said bimetallic support stripsto a respective one of said arcuate shroud segments, whereby saidbimetallic support strips are deflected to move said arcuate shroudsegments inwardly relative to said casing with increasing turbineoperating temperature and outwardly with decreasing temperature.

3. In a turbomachine, a cylindrical casing, a turbine wheel rotatablymounted in said casing, a row of radially extending turbine bucketsperipherally mounted about said turbine wheel, a shroud ringcircumferentially surrounding said row of turbine buckets in spacedrelation thereto, said shroud ring comprised of a plurality of separatecircumferentially extending arcuate segments, first and secondcircumferentially extending flanges extending radially inwardly fromsaid casing and forming therewith a circumferential channel surroundingsaid shroud ring, a first circumferentially extending groove in saidfirst flange opening into said channel, a second circumferentiallyextending groove in said second flange opening into said channel, saidfirst and second grooves being in radial alignment, a plurality ofbimetallic support strips, each of said bimetallic support stripscomprised of two layers having diiferent coeflicients of thermalexpansion, the ends of each of said bimetallic support strips beingreceived in said first and second grooves so that the center of thesupport strip spans said channel with the layer having the lowercoeflicient of thermal expansion adjacent said casing, means connectingthe center portion of each of said bimetallic support strips to arespective one of said arcuate shroud segments, whereby said bimetallicsupport strips are deflected to move said arcuate shroud segmentsinwardly relative to said casing with increasing turbine operatingtemperature and outwardly with decreasing temperature.

4. In a turbomachine, a cylindrical casing, first and secondcircumferentially extending flanges extending radially inwardly fromsaid casing and forming therewith a circumferential channel, acylindrical shroud ring positioned radially inward of said channel, saidshroud ring comprised of a plurality of separate circumferentiallyextending arcuate segments, a first circumferentially extending groovein said first flange opening into said channel, a secondcircumferentially extending groove in said second flange opening intosaid channel, said first and second grooves being in radial alignment, aplurality of bimetallic support strips, each of said bimetallic supportstrips comprised of two layers having different coeflicients of thermalexpansion, the ends of each of said bimetallic support strips beingreceived in said first and second grooves so that the center of thesupport strip spans said channel with the layer having the lowercoeflicient of thermal expansion adjacent said casing, means connectingthe center portion of each of said bimetallic support strips to arespective one of said arcuate shroud segments, whereby said bimetallicsupport strips are deflected to move said arcuate shroud segmentsinwardly relative to said casing with increasing temperature andoutwardly with decreasing temperature.

References Cited in the file of this patent UNITED STATES PATENTS1,761,808 Weaver June 3, 1930 1,857,961 Lamb May 10, 1932 2,253,904 HaugAug. 26, 1941 2,859,934 Halford et al. Nov. 11, 1958 2,962,256 BishopNov. 29, 1960 2,963,307 Bobo Dec. 6, 1960 2,994,472 Botje Aug. 1, 19613,042,365 Curtis et al. July 3, 1962 3,056,583 Varadi et al. Oct. 2,1962 3,085,398 Ingleson Apr. 16, 1963

1. IN A TURBOMACHINE, A CYLINDRICAL CASING, A TURBINE WHEEL ROTATABLY MOUNTED IN SAID CASING, A ROW OF RADIALLY EXTENDING TURBINE BUCKETS PERIPHERALLY MOUNTED ABOUT SAID TURBINE WHEEL, A SHROUD RING CIRCUMFERENTIALLY SURROUNDING SAID ROW OF TURBINE BUCKETS IN SPACED RELATION THERETO, SAID SHROD RING COMPRISED OF A PLURALITY OF SEPARATE CIRCUMFERENTIALLY EXTENDING ARCUATE SEGMENTS, A PLURALITY OF BIMETALLIC SUPPORT STRIPS MOUNTED IN SAID CASING, SAID BIMETALLIC SUPPORT STRIPS EACH COMPRISED OF TWO LAYERS HAVING DIFFERENT COEFFICIENTS OF THERMAL EXPANSION, MEANS CONNECTING THE CENTER PORTION OF EACH OF SAID BIMETALLIC SUPPORT STRIPS TO A RESPECTIVE ONE OF SAID ARCUATE SHROUD SEGMENTS, SAID BIMETALLIC SUPPORT STRIPS DISPOSED SO AS TO DEFLECT WITH TEMPERATURE CHANGES TO MOVE SAID ARCUATE SHROUD SEGMENTS INWARDLY RELATIVE TO SAID CASING WITH INCREASING TURBINE OPERATING TEMPERATURE AND OUTWARDLY WITH DECREASING TEMPERATURE. 